The light response channel of Phycomyces will be studied as a model system for sensory transduction in primary receptor cells. Double-mutant strains have recently been constructed with defects in all pairwise combinations of the seven genes associated with the blue light responses of the sporangiophore. The light growth response is being studied using the white noise method of nonlinear system identification, followed by formal system analysis. Experiments are being performed on the Phycomyces tracking machine. Transfer functions computed from the responses of double-mutant, single-mutant and wild-type strains will be combined mathematically, in order to determine the dynamics and organization of the underlying stimulus-response pathway. In addition mutants with new types of defects will be sought, particularly those with alterations in the presumed flavin photoreceptor. These mutants will be classified genetically and physiologically and then incorporated in the system analysis program. A specialized dual-wavelength spectrophotometer will be used to study light-induced absorbance changes in these mutants. Such absorbance changes will be sought differentially between mutants and wild type to provide a genetic and spectrophotometric assay for the blue-light photoreceptor.